Light emitting diodes (LEDs) are widely accepted in many applications that require low power consumption, small size, and high reliability. Energy-efficient diodes that emit light in the yellow-green to red regions of the visible spectrum contain active layers formed of an AlGaInP alloy. The conventional AlGaInP LED, shown in FIG. 1, includes a semiconductor substrate, e.g. GaAs, a lower confining layer, an active layer, an upper confining layer, all placed in a “double heterostructure” configuration, followed by an optional window layer. The confining layers are made of a transparent semiconductor and enhance the “internal quantum efficiency” of the LED, defined as the fraction of electron-hole pairs in the active layer that recombine and emit light. The window layer, also a transparent semiconductor, increases the spread of electric current across the active layer and enhances the internal quantum efficiency of the diode.
The internal quantum efficiency of an AlGaInP LED depends upon, among other things, the thickness of the active layer and its alloy composition (which determines the color of the emitted light), and the alloy composition of the confining layers. Curve (a) of FIG. 2 shows the internal quantum efficiency of an absorbing substrate AlGaInP LED as the thickness of the active layer is varied. The efficiency of the LED depends on the degree to which the electrons or holes (whichever is the minority carrier) recombine radiatively in the active layer. The alloy compositions and doping concentrations of the confining layers are chosen to create a potential energy barrier between the active and confining layers. A relatively small fraction of the injected minority carriers have sufficient kinetic energy to overcome the barrier and diffuse out of the active layer. Thus, if the active layer thickness is less than the diffusion length of the minority carriers, then the minority carrier concentration is increased by the presence of the confining layers (if a constant current is applied to the device). This results in an increase in internal quantum efficiency, since the rate at which carriers recombine radiatively increases with the carrier density. If the active layer thickness is greater than the diffusion length, then the internal quantum efficiency decreases because the confining layers do not increase the carrier density.
Although the confining layer compositions are selected to maximize the confinement energy, in the AlGalnP material system, this energy is not large enough to completely prevent carriers from “leaking” out of the active layer. In the wide-energy-gap alloys utilized for the confining layers ((AlxGa1−x)0.5In0.5P with x>0.55), non-radiative recombination occurs at a high rate, so carriers which leak out of the active layer are essentially lost and the internal quantum efficiency of the LED suffers. The magnitude of the leakage current is determined by alloy compositions of the active layer and adjacent layers and the resultant differences in their energy gaps. Thus, if a wider-gap active layer is used to generate 590-nm light, the carrier confinement is poorer than if the active layer generates 630-nm light, when the LED is otherwise identical. Those skilled in the art will recognize that LEDs do not emit light of only one wavelength. The LED wavelength is defined at the point of maximum photon emission The rate at which carriers escape from the active layer is furthermore related to the concentration of carriers located at the interface between the active and confining layers. This concentration decreases as the active layer thickness increases. Taking these two effects together (leakage and carrier concentration), the active layer thickness for highest internal quantum efficiency will vary with the color of the emitted light. This is illustrated by curve (b) of FIG. 2, which shows, as a function of active layer thickness, the efficiency of a second AlGaInP LED that emits at a shorter wavelength. Because the confinement energy is smaller, an optimal active layer is thicker.
In U.S. Pat. No. 5,153,889; Sugawara et at. show that if the active layer of an absorbing substrate AlGaInLP LED is thicker than the diffusion length of the injected minority carriers, then the double heterostructure does not provide additional confinement of electrons and holes in the active layer. On the other hand, if the active layer is too thin (<1500 Å, according to the authors), then the density of carriers within the active layer is so high that a substantial fraction of them escape into the confining layers. For a p-type active layer with a net hole concentration of about 5×1016 cm−3 and an upper confining layer composition (Al0.5Ga0.5)0.5In0.5P with a net hole concentration of about 5×1017 cm−3, the optimal active layer thickness is specified to be between 1500 Å and 7500 Å. In U.S. Pat. No. 5,710,440, Kagawa et at. demonstrate that with an (Al0.5Ga0.3))0.5In0.5P upper confining layer with a net hole concentration of about 3×1017 cm−3, the optimum active layer thickness ranges between 1.1 μm and 1.3 μm for an absorbing substrate (AS) LED.
Another way in which the internal quantum efficiency can be improved, particularly for short-wavelength-emitting LEDs, is with multi-quantum-well (MQW) structures. In these devices, the light emission occurs in multiple (usually five or more) thin quantum well active layers of light-emitting AlGaInP (also known as “wells”) between multiple “barrier” layers of another alloy composition of AlGaJnP that is transparent to visible light. An active region consists of one or more light-emitting layers. For a MQW structure, optically transparent higher bandgap barrier layers separate the active layers. The total active region thickness is the sum of the thicknesses of all active layers (wells) and barriers. The total active layer thickness is the sum of the thicknesses of all the individual light emitting active layers (wells). For a single light emitting layer device, the active layer and active region thickness are the same. To form a quantum well (wherein the carriers exhibit quantum size effects), the thickness of the wells must be less than 200 Å, which is roughly the length of the wave function of a thermal electron in AlGaInP, in the effective mass approximation. The exact thickness depends on the alloy composition of the quantum wells and barriers. If the carriers which leak out of the thin quantum well can recombine in a second or third, or fourth, etc. well, the internal quantum efficiency of the LED is improved. It is for this reason that quantum-well LEDs typically have several tens of wells in the active region. Furthermore, the total thickness of the wells is described as the active layer thickness because light is not emitted from the barriers. Sugawara et al. describe in U.S. Pat. No. 5,410,159 a method for determining the optimum combination of well thicknesses and number of wells to produce a high efficiency absorbing substrate LED. Utilizing forty 50 Å-thick wells (for a total active layer thickness of 2000 Å) with alloy composition (Al0.3Gao0.7)0.5In0.5P, the authors achieved ˜2.7% external quantum efficiency at 20 mA of drive current, and an emission wavelength of 575 nm. Huang et al. also describe the use of MQW active regions in U.S. Pat. No. 5,661,742, although the authors do not specify the external quantum efficiencies that they achieved.
Internal quantum efficiency is one factor determining the “external quantum efficiency” of a LED, defined as the ratio of the number of photons exiting the LED to the number of electrons which enter it through the contacts. Another factor is the “extraction efficiency”, defined as the fraction of photons generated in the active layer that escape from the semiconductor surfaces of the LED and enter the surrounding material. The optional window layer enhances extraction efficiency by allowing more light to exit the semiconductor material. The extraction efficiency of an LED can be much improved by either growing or mechanically bonding the lower confining layer upon a transparent substrate (TS) rather than an absorbing one. The extraction efficiency of TS AlGaInP LEDs can be approximately twice as high as that of AS AlGaInP LEDs, improving the external quantum efficiency of the LED by approximately a factor of two.
The extraction efficiency of a transparent substrate LED (TS-LED) is reduced by the presence of any layers in the LED that have an energy gap equal to or smaller than that of the light-emitting layers. This is because some of the light that is emitted by the active layer passes through the absorbing layers before it exits the LED. Typical, but not all, absorbing layers are formed of alloys of (AlxGa1−x)0.5In0.5P where x<0.55, or of AlyGa1−yAs and related alloys. These layers may be located between the active layer and the window layer, and between the lower confining layer and the substrate. These absorbing layers are included because they reduce the number of dislocations or other defects in the active layer or are used to simplify the LED manufacturing process. Another effect is to reduce band offsets at heterointerfaces, which lower the voltage that must be applied to the contacts in order to force a particular current through the diode. Because the absorbing layers tend to absorb shorter-wavelength light more effectively than longer-wavelength light, LEDs that emit at 590 nm suffer a greater performance penalty due to the presence of these layers than LEDs that emit at 640 nm.
Absorption in the active region also reduces the extraction efficiency. FIG. 3 is a diagram of light passing through a transparent substrate AlGaInP LED. The arrow represents the light emitted by the active layer when an electric current is injected at the p-n junction via the contacts of the LED. This ray of light is emitted towards the bottom of the device, reflects off of the back contact, and passes again through the substrate, lower confining layer, and active layer. The active layer reabsorbs part of this ray of light as it passes through, as indicated by the thinning of the line representing the ray of light. The absorption coefficient of the active layer is typically not as large as that of the narrow gap layers in the LED. However, because light rays may reflect off the internal surfaces of the LED several times (and pass through the active layer several times) before escaping, a substantial fraction of the emitted light may be absorbed in the active layer. In contrast, light emitted by the active layer of an absorbing substrate LED only passes through the active layer once, because light that reflects off of an internal surface will generally be completely absorbed by the substrate. Therefore, absorption in the active layer has little effect on the external quantum efficiency of an AS LED.
When light is absorbed by the active layer (either a single layer or a plurality of layers arranged in a multiple-well configuration), electron and hole pairs are formed that may recombine radiatively or non-radiatively. In AlGaInP active layers, only a fraction of the absorbed photons is re-emitted. This fraction is equivalent to the internal quantum efficiency of the active layer, and is determined by the alloy composition of the active layer (i.e. the emission wavelength of the LED) and the predilection of electron-hole pairs to recombine non-radiatively through crystalline defects or impurities. Typically, for a 590 nm LED, only 5–50% of the absorbed photons will be re-emitted by the active layer. Thus, 95–50% of the light that is originally emitted by and subsequently absorbed by the active layer is lost irretrievably, resulting in a decrease in extraction efficiency and the external quantum efficiency of the device.
In the prior art, techniques for improving the efficiency of AlGaInP LEDs have focussed on determining the active layer thickness which results in greatest internal quantum efficiency and on increasing the extraction efficiency of the LED by removing the absorbing substrate.